The present invention relates to a burst signal generating apparatus for use in a time division multiple access (TDMA) communication system and, more particularly, to an apparatus for generating FSK (frequency shift keying) or FM (frequency modulation) burst signals.
FSK or FM burst signal generating apparatuses proposed to date include the "Burst Signal Transmission System" by K. Watanabe, which is the subject matter of the published unexamined Japanese patent application No. 99851/84, assigned to the present applicant; corresponding to which is the U.S. patent application Ser. No. 555,926 filed Nov. 29, 1983, now U.S. Pat. No. 4,592,073. The system proposed by Watanabe includes an FSK or FM modulator and a high power (HP) amplifier comprising a preamplifier and a non-linear power amplifier (Class C amplifier) which are sequentially connected. To expand the dynamic range of the HP amplifier, the preamplifier is prebiased. This permits an FSK or FM burst signal to have gently sloped leading and trailing edges. However, this prebiasing invites leaking of the carrier wave from the modulator to the output of HP amplifier, even during the pauses of burst signals. Therefore, sufficient isolation has to be achieved by a control circuit consisting of an attenuator, which is provided between the modulator and the HP amplifier.
Sufficient suppression of the carrier wave in a control circuit, especially of a carrier wave in the UHF band or an even higher frequency region, is achieved by cascade connection of a plurality of PIN diodes. This connecting structure invites an increase in insertion loss, and has disadvantages in power efficiency and cost.
Furthermore, the reference waveform (burst envelope waveform) generator circuit included in the system proposed by Watanabe is a digital circuit, which successively reads data out of a read-only memory (ROM) by clock pulses and, after converting them into analog signals with a digital/analog converter, filters the converted data to generate the intended envelope. Such a digital envelope generator circuit, since it is generally required to operate at high speed, consumes much power and, as it also needs such peripheral circuits as a latching circuit and a digital/analog converter, requires a large packaging space, resulting in a bulky configuration.